CONCLUDING OBSERVATIONS. 357 
body, and yet it receives about the fifth or sixth part of 
all the blood in circulation, simply because the amount 
of thinking done there requires this supply to keep the 
instrument of thought in good condition. To prevent 
this great amount of blood from flowing too rapidly and 
forcibly into the brain, the arteries, as they enter the 
skull, are so arranged that the flow shall be circuitous 
rather than direct. Then, again, there is a farther spe- 
cial provision against the too free admission of blood into 
the brain in animals that hold their heads downward 
much of the time, as grazing animals. When we hold 
our heads downward, very uneasy sensations are soon 
produced from the undue amount of blood in the head ; 
but in the grazing animal this effect is prevented by a di- 
vision of the arteries into a net-work before they enter 
the brain. In this connection I will also refer you to the 
remarkable provision against a sudden rush of blood to 
the head in the deer when the circulation in the ‘‘ velvet” 
is stopped (§ 164). 
622. The adaptations which we witness in the differ- 
ent conditions of animals that pass through a full meta- 
morphosis are of exceeding interest. That the adapta- 
tions of a crawling worm should all be exchanged, dur- 
ing the sleep of the animal, for those of a beautiful flying 
insect (§ 405), is one of the most wonderful things in na- 
ture. Still more wonderful is the change of adaptation, 
when an animal fitted to live like a fish experiences, in 
the midst of a state of full activity, mternal changes 
which prepare it at length to emerge with lungs and 
wings, leaving its skin behind it in the water, as exempli- 
fied in the musquito (§ 502). 
623. But adaptation is displayed in the most interest- 
ing manner in the relations of organization to the capa- 
bilities of animals. The more an animal knows, the more 
complicated is its structure, or, in other words, the more 
extensive is the machinery which is provided for the use 
of its mind. We see this both in the apparatus of the 
